Do not trim that mustache! It needs to grow longer precisely because it is light-colored and straight (at this length, so far), and then your sister will have to try other ways to test your temper . The philtrum gap in the mustache is also normal. Since the cheeks have gaps between the cheekline and the jaw, I think they will fill in in the next several years. I would suggest keeping the mustache and goatee, which are both strong, until then.